The invention relates to seafloor electromagnetic surveying for resistive and/or conductive bodies, for example for oil and other hydrocarbon reserves or subterranean salt bodies.
FIG. 1 schematically shows a surface vessel 14 undertaking controlled source electromagnetic (CSEM) surveying of a subterranean strata configuration using standard techniques [1]. The subterranean strata configuration in this example includes an overburden layer 8, an underburden layer 9 and a hydrocarbon reservoir 12. The surface vessel 14 floats on the surface 2 of a body of water, in this case seawater 4 of depth h meters. A submersible vehicle 19 carrying a source in the form of a horizontal electric dipole HED transmitter 22 is attached to the surface vessel 14 by an umbilical cable 16. This provides an electrical and mechanical connection between the submersible vehicle 19 and the surface vessel 14. The HED transmitter is supplied with a drive current so that it broadcasts an HED electromagnetic (EM) signal into the seawater 4. The HED transmitter is positioned a height z′ (typically around 50 meters) above the seafloor 6. The EM signals comprise transverse electric (TE) and transverse magnetic (TM) mode components.
One or more remote receivers 25 are located on the seafloor 6. Each of the receivers 25 include an instrument package 26, a detector 24, a floatation device 28 and a ballast weight (not shown). The detector 24 comprises an orthogonal pair of horizontal electric dipole detectors and an orthogonal pair of horizontal magnetic field detectors positioned a height z above the seafloor 6. The horizontal electric dipole detectors are sensitive to horizontal components of the electric fields induced by the HED transmitter in the vicinity of the receiver 25, and produce electric field detector signals therefrom. The horizontal magnetic field detectors are sensitive to horizontal components of the magnetic fields, for example the magnetic flux density, induced by the HED transmitter in the vicinity of the receiver 25, and produce magnetic field detector signals therefrom. The instrument package 26 records the detector signals for later analysis. Examples of suitable receivers are described by Constable [8] and U.S. Pat. No. 5,770,945 [9].
The HED transmitter 22 broadcasts EM signals that propagate outwards both into the overlying water column 4 and downwards into the seafloor 6 and the underlying strata 8, 9, 12. At practical frequencies for this method and given the typical resistivity of the respective media 4, 8, 9, 12, propagation occurs by diffusion of electromagnetic fields. The rate of decay in amplitude and the phase shift of the signal are controlled both by geometric spreading and by skin depth effects. Because in general the underlying strata 8, 9, 12 are more resistive than the seawater 4, skin depths in the underlying strata 8, 9, 12 are longer. As a result, electromagnetic fields measured by a receiver located at a suitable horizontal separation are dominated by those components of the transmitted EM signal which have propagated downwards through the seafloor 6, along within the underlying strata 8, 9, 12, and back up to the detector 24 rather than directly through the seawater 4.
A sub-surface structure which includes a hydrocarbon reservoir, such as the one shown in FIG. 1, gives rise to a measurable increase in the horizontal electric field component amplitudes measured at the receiver relative to a sub-surface structure having only water-bearing sediments. This is because hydrocarbon reservoirs have relatively high resistivities (typically 100 Ωm) compared to other subterranean strata (typically 1 Ωm) and so the EM signals are less attenuated. It is this enhancement in horizontal electric field amplitudes which has been used as a basis for detecting hydrocarbon reservoirs [1].
It is important when surveying for hydrocarbon reservoirs to carefully consider the orientation of the current flows induced by a transmitted EM signal. The response of seawater and subterranean strata (which will typically comprise planar horizontal layers) to EM signals is generally very different for TE mode components of the transmitted signal, which excite predominantly horizontal current flows, and TM mode components, which excite significant components of vertical current flow.
For TE mode components, the coupling between the layers comprising the subterranean strata is largely inductive. This means the presence of thin resistive layers (which are indicative of hydrocarbon reservoirs) does not significantly affect the EM fields detected at the surface as the large scale current flow pattern is not affected by the thin layer. On the other hand, for TM mode components, the coupling between layers includes a significant galvanic component (i.e. due to the direct transfer of charge between layers). For the TM mode even a thin resistive layer strongly affects the EM fields detected at the receiver since the large scale current flow pattern is interrupted by the resistive layer. It is known therefore that a significant component of the TM mode is required to satisfactorily perform an EM survey in the field of oil exploration.
However, sole reliance on the sensitivity of the TM mode components to the presence of a thin resistive layer can lead to ambiguities. The effects on detected EM fields arising from the presence a thin resistive layer can be indistinguishable from the effects arising from other realistic large scale subterranean strata configurations. In order to resolve these ambiguities it is known to determine the response of the subterranean strata to both TM mode components (i.e. inductively coupled) and TE mode components (i.e. galvanically coupled) [1].
The HED transmitter 22 shown in FIG. 1 simultaneously generates both TE and TM mode components with the relative contribution of each mode to the signal at the receiver depending on the HED transmitter-receiver orientation. At receiver locations which are broadside to the HED transmitter axis, the TE mode dominates the response. At receiver locations which are inline with the HED transmitter axis, the TM mode is stronger (although the TE mode is still present) [1, 2, 3, 4]. The response at receiver locations in both the inline and broadside configurations is governed by a combination of the TE and TM mode components, and these tend to work in opposition.
Previous surveys [5, 6] have relied on this geometric splitting of the TE and TM mode components to determine the different response of the subterranean strata to the different modes. This is achieved by collecting electric field amplitude data for different transmitter-receiver alignments. This approach provides complementary horizontal electric field amplitude data sets which are differently sensitive to the TE and TM mode components of the transmitted EM signals. During analysis, these complementary data sets are combined to reveal differences between the TE mode and TM mode coupling between the transmitter and the receiver. These differences are indicative of the presence or not of a subterranean hydrocarbon reservoir. Because of the need to survey with multiple transmitter-receiver alignments, this approach requires a relatively large numbers of tow lines and receivers to ensure adequate coverage.
FIG. 2 shows in plan view an example survey geometry for collecting horizontal electric field component data to be analysed according to known methods. Sixteen receivers 25 are laid out in a square grid on a section of seafloor 6 above a subterranean reservoir 56 having a boundary indicated by a heavy line 58. The orientation of the subterranean reservoir is indicated by the cardinal compass points (marked N, E, S and W for North, East, South and West respectively) marked in the upper right of the figure. To perform a survey, a transmitter starts from location ‘A’ and is towed along a path indicated by the broken line 60 through location ‘B’ until it reaches location ‘C’ which marks the end of the survey path. As is evident, the tow path first covers four parallel paths aligned with the North-South direction to drive over the four “columns” of the receivers. This part of the survey path moves from location ‘A’ to ‘B’. Starting from location ‘B’, the survey path then covers four paths aligned with the East-West direction which drive over the four “rows” of receivers. Each receiver is thus driven over in two orthogonal directions. The survey is completed when the transmitter reaches the location marked ‘C’.
During the towing process, each of the receivers 25 presents several different orientation geometries with respect to the transmitter. For example, when the transmitter is directly above the receiver position D1 and on the North-South aligned section of the tow path, the receivers at positions D5, D6 and D7 are at different separations in an inline position (i.e. aligned with the dipole axis of the HED transmitter), the receivers at positions D2, D3 and D4 are at different horizontal separations in a broadside position and the receiver at positions D8 and D9 are inbetween. However, when the transmitter later passes over the receiver position D1 when on the East-West aligned section of the tow path, the receivers at positions D5, D6 and D7 are now in a broadside position, and the receivers at position D2, D3 and D4 are in an inline position. Thus, in the course of a survey, and in conjunction with the positional information of the transmitter, data from the receivers can be used to provide details of the signal transmission through the subterranean strata for a comprehensive range of distances and orientations between transmitter and receiver, each with varying TM mode and TE mode contributions to the signal propagation.
In addition to requiring relatively complex tow paths, another problem with known survey and analysis techniques is they do not provide good results for surveys made in shallow waters. This is due to the presence of an ‘airwave’ component in the EM fields induced by the HED transmitter at the receiver. This airwave component is due to EM signals from the HED transmitter which interact with the air. Since air is non-conducting and hence causes little attenuation, the airwave component can dominate the fields at the receiver. The airwave component is principally due to the TE mode components. This is because the TE mode components are efficiently inductively coupled across the seawater-to-air interface. The TM mode components, on the other hand, do not couple well across this boundary and consequently do not contribute significantly to the airwave component. The airwave component contains little information about subterranean resistivity. Accordingly, if the airwave contributes a significant component to the EM fields induced by the HED transmitter at the receiver, the sensitivity of the technique to subterranean resistivity structures, such as hydrocarbon reservoirs, is greatly reduced. The path of an example airwave component is shown in FIG. 1 by a dotted line labelled AW. The magnitude of the airwave component is reduced only by geometric spreading. This is because air is non-conducting. However, as with other components, the airwave component is strongly attenuated by its passage through the seawater. This means that in relatively deep water (large h) the airwave component is not very significant at the receiver and as such does not present a major problem. However in shallow water (small h) the airwave component does not pass through as much seawater and thus makes a larger contribution to the EM fields induced by the HED transmitter at the receiver. This contribution becomes greater still at increasing transmitter-receiver horizontal separations. This is because (other than due to geometric spreading) the strength of the airwave component is relatively constant over a wide range of horizontal separations since any extra distance traveled by the airwave component is almost exclusively in the non-attenuating air. Other components of the EM fields induced by the HED at the receiver, such as those which pass through the subterranean strata and are of interest, travel through lower resistivity media and become increasing attenuated as they travel further. For these reasons, the airwave component tends to dominate the EM fields induced by the HED transmitter at the receiver for surveys made in shallow water, especially at long transmitter-receiver horizontal separations.
The existence of the airwave as a dominant component of the detector signals limits the applicability of the above described surveying and analysis techniques. In shallow water the transmitter-receiver separations over which the techniques can be applied is much reduced. This not only leads to a need to employ more receiver locations to adequately cover a given area, but also limits the depth beneath the seafloor to which the technique is sensitive. This can mean that a buried hydrocarbon reservoir in shallow water may not be detectable, even though the same reservoir would be detected in deeper water.
FIG. 3A is a graph schematically showing results of one-dimensional modelling of two example EM surveys of the kind shown in FIG. 1. One example corresponds to a survey performed in deep water (dotted line) and the other to a survey performed in shallow water (solid line). For each model survey the amplitude of an electric field component induced at the receiver in response to the HED EM transmitter is calculated per unit transmitter dipole moment and is plotted as a function of horizontal separation r between the HED transmitter and the receiver. For both model surveys, the subterranean strata configuration is a semi-infinite homogeneous half space of resistivity 1 Ωm. In the deep-water example, the subterranean strata configuration is located beneath an infinite extent of seawater. In the shallow-water example, it is located beneath a 500-meter depth of seawater. In both cases the seawater has resistivity 0.3 Ωm. The transmitter and receiver are separated along a line which runs through the axis of the HED transmitter (inline orientation). It is the component of detected electric field resolved along this direction which is plotted in FIG. 3A. The HED transmitter is driven by an alternating current (AC) drive signal at a frequency of 0.25 Hz.
The effect of the airwave component on the amplitude of EM fields induced by the HED transmitter at the receiver is clear. In the deep-water model survey, where there is no airwave component, the calculated electric field amplitude falls steadily with increasing horizontal separation. In the shallow-water model, however, where there is a strong airwave component, the rate of amplitude reduction sharply decreases at a transmitter-receiver horizontal separation of about 5000 m. FIG. 3B is a plot showing the ratio, p, of the two curves shown in FIG. 3A. The large deviations from unity seen in FIG. 3B highlight the difference between these curves. Since the only difference between the two model surveys is the presence or not of an airwave component, the ratio plotted in FIG. 3A effectively shows the relative strength of the airwave component in the detected signal compared to that which passes through the subterranean strata for the shallow-water model survey.
It is apparent from FIGS. 3A and 3B that at all but the very shortest horizontal separations the detected electric field is significantly larger in the shallow-water model. For example, at a horizontal separation of 2500 m, the amplitude of the detected signal in the deep-water model survey is around 10−12 V/Am2. In the shallow-water model survey it is higher at around 10−11.5 V/Am2. This is due to the additional contribution of the airwave component. This level of increase shows that the airwave component has an amplitude more than double that of the component which has passed through the subterranean strata, and accordingly over two-thirds of the detector signal carries almost no information about the subterranean strata. At greater horizontal separations the airwave component dominates even more. In particular, it becomes especially pronounced beyond around 5000 m. At this point there is a break in the rate at which the detected electric field amplitude falls with increasing horizontal separation. At a horizontal separation of around 7000 m, the airwave component in the shallow-water example has an amplitude around twenty times greater than that of the signal which passes through the subterranean strata. This clearly imposes high requirements for the signal-to-noise ratio of data collected over these sorts of horizontal separations, as is generally the case when a small signal rides on a large background. It is apparent that the airwave significantly limits the usefulness of these surveying and analysis techniques in shallow water.
While this survey method has been demonstrated to provide good results in practice, as noted above some limitations have been identified.
Firstly, since the TE and TM mode components cannot be easily separated there will generally be a level of cross-talk between them at a receiver. This may lead to ambiguities in the results.
Secondly, in order to obtain survey data from both inline and broadside geometries, the HED transmitter needs to be re-oriented at each HED transmitter survey location. This requires the surface vessel to make multiple passes over broadcast locations and can lead to complex and long tow patterns.